Group III nitride semiconductors offer a direct transition over a band gap energy from visible light to ultraviolet rays, and excel in the light emission efficiency, and thus have been manufactured as semiconductor light-emitting devices such as a light emitting diode (LED) and a laser diode (LD) for use in various applications. In addition, when used for an electronic device, Group III nitride semiconductors have a potential to provide electronic devices having characteristics superior to those using conventional Group III-V compound semiconductors.
Such Group III nitride compound semiconductors are, in general, produced from trimethyl gallium, trimethyl aluminum, and ammonia as raw materials through a Metal Organic Chemical Vapor Deposition (MOCVD) method. The MOCVD method is a method in which a vapor of a raw material is introduced into a carrier gas to convey the vapor to the surface of a substrate and decompose the raw material due to the reaction with the surface of the heated substrate, to thereby grow a crystal. Meanwhile, hitherto, a single crystal wafer of a Group III nitride semiconductor has not been commercially available, and Group III nitride semiconductors are, in general, produced by growing a crystal on a single crystal wafer of a different material.
As the above method for growing a Group III nitride semiconductor, there has been proposed and generally performed a method for epitaxially growing a Group III nitride semiconductor crystal on a single crystal sapphire substrate or a single crystal SiC substrate through a Metal Organic Chemical Vapor Deposition (MOCVD) method, in which, firstly, a layer called a low temperature buffer layer made of aluminum nitride (AlN) or aluminum nitride gallium (AlGaN) is laminated on a substrate, and then a Group III nitride semiconductor crystal is epitaxially grown thereon at a high temperature (for example, Patent Documents 1 and 2).
In addition, there has been proposed the technique for forming the buffer layer through a method other than the MOCVD method. For example, a method has been proposed in which a buffer layer is formed by high frequency sputtering, and a crystal having the same composition is grown thereon by the MOCVD method (for example, Patent Document 3).
In addition, research has been conducted on the manufacture of a Group III nitride semiconductor crystal by a sputtering method. For example, with a purpose of laminating high resistance GaN, a method for forming a GaN film directly on a substrate made of sapphire by a sputtering method has been proposed (for example, Patent Document 4). The film formation of GaN using a sputtering method has advantages in that facilities are inexpensive in comparison with the film formation using a MOCVD method as described in the above Patent Documents 1-3 and that the production yield is improved due to the stabilization of each step.
When a crystal of a Group III nitride semiconductor is subjected to the film formation using a sputtering method, a substrate temperature during the film formation is important parameter. As a result of intensive studies conducted by the inventors of the present invention, a film of GaN having excellent crystallinity is formed by setting a substrate temperature to a relatively high temperature when GaN is subjected to the film formation using a conventional sputtering method as described in the above Patent Document 4. However, the surface of a film gets rough, and the film formation rate extremely decreases, to thereby lowering the production yield. Meanwhile, when the film formation is performed while setting a substrate temperature to a relatively low temperature, the film formation rate is improved. However, the crystallinity of the formed film of GaN deteriorates. In particular, it was found that the full width at half maximum in the X-ray rocking curve of the asymmetric plane increased.
Therefore, a method has been desired which is capable of stably forming a film having excellent crystallinity on a substrate with high efficiency when a Group III nitride semiconductor is formed by using a sputtering method.
Patent Document 1: Japanese Patent No. 3026087
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. Hei 4-297023
Patent Document 3: Japanese Examined Patent Application, Second Publication No. Hei 5-86646
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. Sho 60-039819